1. Field of the Invention
The present invention relates to a medial image processing apparatus and a medical image processing method for generating a three-dimensional medical image used in image diagnosis, etc. and particularly relates to a medical image processing apparatus, and a medical image processing method suitable for displaying a tubular tissue such as a vessel.
2. Description of the Related Art
Conventionally, there has been conducted image diagnosis for diagnosing diseases, etc. by acquiring a tomographic (slice) image of an organ, etc. of a human body using a modality (image acquiring apparatus) such as a CT scanner, an MRI apparatus, etc. Further, a method of conducting diagnosis by generating a three-dimensional image of a predetermined organ, etc. based on an acquired tomographic image has been established, contributing to a precise and accurate diagnosis. Especially, diagnosis by a three-dimensional image of a coronary vessel around a heart is helpful for an early detection of a heart disease.
In such three-dimensional image diagnosis, it is often the case that an image of an observation target part is extracted from an original three-dimensional image and then diagnosed by an observation. However, in general, it was difficult to extract an image of an elongated tubular tissue such as a vessel (particularly, a coronary vessel around a heart), in the past.
To solve such a problem, a method for an extraction of an image of a vessel tissue is disclosed in the Unexamined Japanese Patent Application KOKAI Publication No. 2002-282235 (pp. 4-5). According to this document, a “binary mask” which separates picture element values indicating a concerned area (i.e. a target vessel tissue) from picture element values indicating areas other than the concerned area is generated. By using this “binary mask”, the vessel tissue is distinguished from other tissues. The picture element value indicates the gradation of a picture element. Further, picture elements include two-dimensional picture elements (pixels) and three-dimensional picture elements (voxels). Hereinafter, these will be denoted by “picture element”.
In a method using a binary mask described above, once a picture element value for indicating a target tissue is set, it is determined whether a given part belongs to the target tissue or not based on whether each value of the picture element in the area subjected to the determination is the value indicating the target tissue. In this case, if only the target vessel for the extraction exists in the area subjected to the determination, the image of the target vessel can be extracted without any trouble. However, in general, other tissues (a vessel, an organ, etc.) exist closely to the target vessel. In particular, in a case where the target is a coronary vessel around a heart, the heart and many other vessels exist near the coronary vessel. Since the compositional elements of those close organs are almost the same as those of the target vessel, the picture element values indicating the close organs are similar to the picture element values indicating the target vessel. Because of this, the target vessel can not be extracted accurately by using a method disclosed in the Unexamined Japanese Patent Application KOKAI Publication No. H8-89501 (p. 6), for the target vessel can not be distinguished from the other tissues.
In a case where a coronary vessel extending from a heart is imaged (visualized) based on CT values obtained by an image acquiring apparatus such as a CT scanner, the generated image is normally reduced in brightness gradually as the coronary vessel extends away from its originating position. That is, it can be said that approximation is established such that picture element values change generally linearly in accordance with the distances from the originating position of the coronary vessel. By utilizing this characteristic, U.S. Pat. No. 6,424,732 discloses a method for extracting a target vessel by using a “multi-value mask” which allocates picture element values to the vessel by changing the values linearly from the originating position of the vessel to the extraction end position. However, in a case where a heart or other vessels exist closely to the target vessel or in a case where the target vessel has an abnormal part, the picture element values indicating the target vessel do not change simply linearly. Consequently, if an image is generated by changing picture element values simply linearly, the resulting image might be such that the target vessel and its close organ are combined or the target vessel is discontinuous. Therefore, it is difficult to obtain an image that can contribute to an accurate diagnosis.
A patent document 2 discloses a method of detecting a vessel part by detecting the average density around the attention point. However, in a case where a heart or other vessels exist closely to the target vessel, an effective density value can not be obtained. Therefore, this method can not extract the vessel part accurately, either.